1) Field of the Invention
The present invention relates to a thermal transfer image-receiving sheet. More particularly, the present invention relates to a sheet for recording thereon images of a thermo-melted or thermal-adhesive ink in a single color or multicolor with a high image sharpness, a high resolution, and at a high reproductivity.
2) Description of the Related Arts
It is known that a new type of printer having a thermal head enables a printing of clear color images or pictures by a thermal transfer of the color images or pictures of a thermo-melting ink or sublimating dye onto a recording sheet, and there is great interest in a further development and utilization of this printing system.
The thermo-melting ink type printer is operated in a manner such that an ink image-receiving sheet is superposed on an ink sheet consisting of a film substrate and a thermo-melting ink layer formed on a surface of the film substrate so that the ink image-receiving sheet comes into contact with the thermo-melting ink layer and the thermo-melting ink layer is partly heated by a thermal head in accordance with electric signals corresponding to the images or pictures to be printed, to thermally transfer the thermo-melting ink images or pictures to the receiving sheet.
In a color printer, color images or pictures are formed by superposing thermally transferred yellow, magenta, cyan and/or black colored images or pictures.
The thermal transfer system using a thermo-melting ink or thermal-adhesive ink is most useful as a small size non-impact printer, for printing Chinese characters and color images.
In the above circumstances, a recording sheet capable of clearly receiving images or pictures thereon is urgently required, together with a clear image-transferring system.
When a record sheet capable of receiving colored images at a high resolution and a high reproductivity is provided, it will become possible to provide hard copies of color pictures, for example, video or computergraphic colored pictures, by a compact printing machine. Accordingly, this type of recording sheet is expected to be widely utilized for many purposes.
The clarity of the images or pictures printed by the thermo-melting ink image-transferring printer depends on the property of the image-receiving sheet, i.e., only an image-receiving sheet compatible with the type of printer used can record high quality clear images: Namely, the properties imparted to the image-receiving sheet should correspond to the type of printer to be used. For example, when the surface of a conventional fine paper having a Bekk smoothness of 10 to 50 seconds is smoothed with a super-calender or other surface smoothing machine to provide an enhanced Bekk smoothness of 100 seconds or more, the resultant paper sheet can receive thermally transferred ink images having an increased clarity.
In the usual thermal transfer system, the images or pictures are presented in a shade (or single color depth), but recently it has become possible to print continuous tone color images by using a dot-pattern (Dizza) type thermal transfer imaging system.
When the Dizza method is used, the thermal transfer printer must be able to transfer images at a resolution of 16 to 32 dots/mm, and at a higher resolution than picture elements.
Very recently, a new dot area type printer which can control the size of dots to be continuously transferred to an accuracy of 10 .mu.m or less has been reported, and this means that the printer must have an even more enhanced resolving power.
In view of the above-mentioned requirement, it is clear that the conventional fine paper sheet in which cellulose fibriles are exposed on the surface of the sheet is not satisfactory as an image-receiving sheet for current thermal transfer type printers.
To improve the thermal-transferring properties of the thermal transfer type printer, Japanese Unexamined Patent Publication No. 57-182,487 discloses an image-receiving sheet having a coating layer containing an oil absorption pigment and formed on a substrate sheet. This type of image-receiving sheet can receive thermally transferred images having a uniform shade. The pigment usable for the above-mentioned coating layer can be selected from among almost all of those usable for usual coated printing paper sheets but when the printer has a high resolution, the reproductivity of the thermally transferred images is greatly influenced by the evenness of the ink-receiving property of the image-receiving layer of the image-receiving sheet. Accordingly, the use of a conventional coated or non-coated image-receiving sheet sometimes results in an uneven shading of close printed portions and in an unstable transfer of dots, and thus preferable continuous tone images are not obtained with this sheet.
Also, it is clear that the usual fine paper sheet or coated paper sheet is not suitable as a thermally transferred image-receiving sheet able to receive red, blue, and green colored images formed from superposed yellow, cyan and magenta colored dots, with a required evenness and at a high reproductivity.
In the above-mentioned thermal transfer printing system, the conventional thermally transferred bi-level images can be replaced by continuous tone type images.
The density modulation continuous tone type printing systems include a system in which sublimating dyes are utilized and the amounts of the sublimated dyes are controlled by controlling the heat quantity in the thermal head, and a new system of thermo-melting inks is utilized, and the amounts of the inks to be transferred are controlled by adding a filler to the ink layers or by multilayer-coating a plurality of inks having different melting points. The latter is a thermo-melting, density modulation thermal transfer imaging system and is expected to be most suitable for the required purpose.
In this system, a specific ink sheet is used in the printer, ink images are transferred to the surface of the image-receiving sheet by heating and pressing the ink sheet by the thermal head toward the image-receiving sheet. The heating is carried out stepwise to transfer a predetermined amount of the ink to the image-receiving sheet and to form full color images having a density modulation on the sheet.
In the sublimating dye-thermal transfer system, an ink sheet consisting of a thin paper or film substrate and an ink layer containing a sublimating dye and formed on the substrate, is superposed on an image-receiving sheet having a polyester resin layer formed on a sheet substrate so that the ink layer comes into contact with the polyester resin layer, and the dye images are transferred from the ink layer to the polyester resin layer by heating the ink layer by the thermal head in accordance with electric signals. The heating is controlled stepwise to transfer a predetermined amount of the dye and to form images having a predetermined shade.
The evenness and clarity of the dye images are influenced by the quality of the image-receiving sheet, and the quality of the image receiving sheet depends on the quality of the image-receiving layer and the quality of the sheet substrate.
It is known that a biaxially and/or uniaxially oriented, multilayered film consisting of at least one polyolefine resin and at least one inorganic pigment, and a composite sheet consisting of the above-mentioned film and an image-receiving layer formed on the film, are useful as a thermal transfer image-receiving sheet.
The image-receiving sheet for sublimating dyes has an image receiving polyester resin layer formed on the polyolefin resin film. This image-receiving sheet has a uniform thickness, a greater softness and a lower heat conductivity than that of the cellulosic pulp sheet (paper), and thus is advantageous in that the received images have a uniform quality and an excellent shade and color depth.
When a biaxially and/or uniaxially oriented multilayered film consisting essentially of a polypropylene resin is used as an image-receiving sheet for the thermal transfer type printer, the heating by the thermal head of the printer causes the sheet to be partially shrunk, and thus to be curled and/or wrinkled. Sometimes the sheet is partially melted, and thus the travel of the sheet in the printer is obstructed and the resultant print becomes useless. Especially, in the sublimating dye-thermal transfer type printer, the necessary amount of heat to be applied to the ink sheet is large, and thus the polyolefin resin film sheet has serious problems in practical use.
To eliminate the above-mentioned problem, Japanese Unexamined Patent Publication No. 62-21590 discloses an image receiving polyolefin resin sheet having a barrier layer formed on a paper sheet substrate and consisting of an organic polymer material having a higher heat resistance than that of the polyolefin resin. This type of image receiving sheet is disadvantageous in that the evenness of dots formed by the thermal head is greatly affected by the surface smoothness of the sheet, and the resultant images have an uneven shading.
The surface smoothness of the image receiving sheet depends greatly on the surface smoothness of the sheet substrate, and accordingly, the paper sheet substrate must have a satisfactory surface smoothness.
The surface smoothness of the paper sheet depends on the type of pulp, the type of pulp treating method, the type of additives used, the paper-forming conditions, and the post treatment conditions. If a paper sheet having an excellent surface smoothness is provided, the paper sheet can be utilized as a sheet substrate of an image receiving sheet having a thermoplastic resin coating layer formed thereon. The surface smoothness of the thermoplastic resin coating layer depends on not only the type of the resin and the coating conditions but also the smoothness of the sheet substrate. The surface smoothness of the image-receiving sheet necessary for obtaining a high evenness of dots on the surface must be considered in a micrographic area, and the usual Bekk smoothness is not always sufficient to define the necessary surface smoothness.
Furthermore, in the sublimating dye image-receiving sheet, colored images are formed by superposing yellow dye images, magenta dye images and cyan dye images. When the colored image-thermal transferring operation is carried out in a low humidity atmosphere, sometimes static electric charges are generated on the sheet and obstruct the travel of the sheet.